1. Field of the Invention
The present invention relates to a wireless communication system, and more particularly to a receiving apparatus having a plurality of antennas.
2. Description of the Related Art
Extensive research and development efforts are under way with regard to reception processes for coded MIMO (Multiple Input Multiple Output) systems. One of those reception processes is based on a combination of a complexity-reduced MLD (Maximum Likelihood Detection) process, which has reduced the calculation complexity of the MLD process, and soft-decision decoding, for achieving simple, high-performance reception characteristics. The method disclosed in Hiroyuki KAWAI, Kenichi HIGUCHI, Noriyuki MAEDA, Mamoru SAWAHASHI, Takumi ITO, Yoshikazu KAKURA, Akihisa USHIROKAWA, Hiroyuki SEKI, “Likelihood Function for QRM-MLD Suitable for Soft-Detection Turbo Decoding and Its Performance for QFCDM MIMO Multiplexing in Multipath Fading Channel,” IEICE TRANS. Commun., Vol. E88-B, No. 1, January 2005 will be described below with reference to FIG. 1 of the accompanying drawings. For the sake of brevity, it is assumed that propagation paths between receiving and transmitting apparatus comprise flat-fading channels.
FIG. 1 is a block diagram of a conventional wireless communication system for carrying out the method disclosed in the above document.
As shown in FIG. 1, transmitting apparatus 11 has three antennas 21-1, 21-2, 21-3, and receiving apparatus 10 has three antennas 11-1, 11-2, 11-3. Receiving apparatus 10 comprises channel estimator 3111, QR decomposition MLD device 3121, bit likelihood calculator 101, and decoder 3123.
Transmitting apparatus 11 is supplied with three signals d1, d2, d3, generates three transmission signals s1, s2, s3 from supplied signals d1, d2, d3, and sends generated transmission signals s1, s2, s3 through respective antennas 21-1, 21-2, 21-3. Antennas 11-1, 11-2,11-3 receive respective signals r1, r2, r3 and send received signals r1, r2, r3 to receiving apparatus 10.
Channel estimator 3111 is supplied with received signals r1, r2, r3, estimates transmission paths between transmitting apparatus 11 and receiving apparatus 10 therefrom, and outputs estimated channel values h11, h12, h13, h21, h22, h23, h31, h32, h33 where hij represents a propagation path provided by antenna 21-j and antenna 11-i. 
QR decomposition MLD device 3121 is supplied with received signals r1, r2, r3 and estimated channel values h11, h12, h13, h21, h22, h23, h31, h32, h33, calculates symbol candidates and likelihoods of the symbols, and outputs symbol candidate and likelihood pairs (S1, e1) . . . (S256, e256) where S1through S256 represents symbol candidates and e1 through e256 symbol likelihoods, the number of symbol candidates being 256.
Bit likelihood calculator 101 is supplied with symbol candidate and likelihood pairs (S1, e1) . . . (S256, e256), calculates bit likelihood pairs (L01, L11), (L02, L12), (L03, L13), and outputs calculated bit likelihood pairs (L01, L11), (L02, L12), (L03, L13).
Decoder 3123 is supplied with bit likelihood pairs (L01, L11), (L02, L12), (L03, L13), decodes bit likelihood pairs (L01, L11), (L02, L12), L03, L13), and outputs decoded data d1, d2, d3.
Bit likelihood calculator 101 will be described in detail below. Bit likelihood calculator 101 calculates a likelihood that the transmitted bit is 0 and a likelihood that the transmitted bit is 1.
FIG. 2 of the accompanying drawings is a block diagram of bit likelihood calculator 101. As shown in FIG. 2, bit likelihood calculator 101 comprises averager 1011, buffer 1012, and selectors 3221-1, 3221-2, 3221-3.
Averager 1011 is supplied with symbol candidate and likelihood pairs (S1, e1) . . . (S256, e256). If both symbol candidates, where each of the bits included in the transmitted three signals is 0, and symbol candidates where each of the bits included in the transmitted three signals is 1, can be selected, then averager 1011 selects the maximum likelihoods of the symbol candidates, averages smaller ones of likelihoods that the bit is 0 and likelihoods that the bit is 1, and outputs average value q.
Buffer 1012 is supplied with symbol candidate and likelihood pairs (S1, e1) . . . (S256, e256) for buffering, and stores supplied symbol candidate and likelihood pairs (S1, e1) . . . (S256, e256) until the averaging process in averager 1011 is finished.
Each of selectors 3221-1, 3221-2, 3221-3 is supplied with symbol candidate and likelihood pairs (S1, e1) . . . (S256, e256) that have been buffered by buffer 1012 and average value q. For calculating a bit likelihood, each selector selects a symbol candidate where each bit is 0 and a symbol candidate where each bit is 1, selects a maximum symbol likelihood of the symbol candidates, and outputs the selected maximum symbol likelihood as a bit likelihood. If there are no symbol candidates including bits 0 or bits 1 and hence each selector is unable to select a bit likelihood, then the selector uses supplied average value q as a bit likelihood of bit 1.
Bit likelihood calculator 101 can widen an averaging interval in averager 1011 to increase the averaging accuracy.
However, the conventional scheme has suffered the following problems:
The first problem is that the scheme causes a large processing delay because of the buffering of the data until the averaging process is over.
Accordingly, it has been difficult to apply the conventional scheme to wireless communication systems which pose strict limitations on any delay times.
The second problem is that the number of samples used for averaging cannot be determined in advance because it is not possible to determine in advance how many symbols that have both bit 0 and bit 1 are present among symbol characteristics that are supplied for the averaging process.